(1) Field of the Invention
The present invention generally relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device which is appropriate to provide good passband characteristics in a wide range of frequencies up to the order of some gigahertz.
(2) Description of the Related Art
Surface acoustic wave devices (which will be called SAW devices) are widely used as filters or resonators in high-frequency circuits of wireless communication systems. In particular, wireless communication systems, such as handy telephones, which are lightweight, portable and operable in a desired high frequency range, utilize the SAW devices as the filters or the resonators.
Generally, single crystal or polycrystal piezoelectric substrates are used as the materials of the substrates of the SAW devices. In particular, a 64.degree.Y-X LiNbO.sub.3 single crystal substrate (K. Yamanouchi and K. Shibayama, J. Appl. Phys., vol.43, no.3, March 1972, pp.856) and a 36.degree.Y-X LiTaO.sub.3 single crystal substrate are known. In these substrates, the crystal has X, Y and Z axes, the X axis being oriented in a direction of propagation of surface acoustic waves, the Y axis being oblique to a perpendicular line normal to a cut plane of the crystal, the cut plane being rotated around the X axis at a rotated angle from the Y axis to the Z axis. In the case of the LiNbO.sub.3 substrate, the rotated angle is set at 64.degree.. In the case of the LiTaO.sub.3 substrate, the rotated angle is set at 36.degree..
In the above-mentioned piezoelectric substrates, the electromechanical coupling coefficient is large, the efficiency of excitation of the surface acoustic waves is high, and the propagation loss in the high-frequency range is small. It is estimated that the operating characteristics of the SAW device in which electrodes, such as interdigital transducers (IDT), are formed on the cut plane of the piezoelectric substrate can be made optimum (the propagation loss being minimized) by setting the rotated angle of the crystal of the substrate at the above-mentioned angle (or 64.degree. in the case of the LiNbO.sub.3 substrate and 36.degree. in the case of the LiTaO.sub.3 substrate).
However, the SAW filter using the conventional piezoelectric substrate does not show good operating characteristics unless the effect of the mass of the electrodes (IDT) formed on the piezoelectric substrate is negligible. The effect of the mass of the electrodes (IDT) on the substrate detrimental to the operating characteristic of the SAW filter is negligibly small when the passband of the SAW filter is relatively low and of the order of some hundred megahertz (MHz). When the passband of the SAW filter is higher than the order of some hundred megahertz and rises to the order of some gigahertz (GHz), which is required for recent handy telephones, the effect of the mass of the electrodes becomes considerable and not negligible. The wavelength of the excited surface acoustic waves in such a case is extremely small, and the thickness of the electrodes relative to the wavelength of the excited surface acoustic waves cannot be ignored. The operating characteristics of the SAW filter of this type are not necessarily optimum when the passband of the SAW filter is of the order of some gigahertz.
When the passband of the SAW filter is of the order of some gigahertz, the passband frequencies of the SAW device can be lowered by increasing the thickness of the electrodes on the substrate so as to make the electromechanical coupling coefficient apparently increase. However, by such a modification, bulk waves radiated from the electrodes into the substrate are increased, and the loss of propagation of the surface acoustic waves is increased. Such bulk waves are called the surface skimming bulk waves (SSBW), and the surface acoustic waves when the bulk waves are produced are called the leaky surface acoustic waves (LSAW) for the SSBW. Some analyses on the propagation loss of the LSAW in the surface acoustic wave filters using the thickened electrodes on the 64.degree.Y-X LiNbO.sub.3 substrate and the 36.degree.Y-X LiTaO.sub.3 substrate are known (for example, V. S. Plessky and C. S. Hartmann, Proc. 1993 IEEE Ultrasonic Symp., pp.1239-1242; P. J. Edmonson and C. K. Campbell, Proc. 1994 IEEE Ultrasonic Symp., pp.75-79).
Further, in the literature by M. Ueda et al (Proc. 1994 IEEE Ultrasonic Symp., pp.143-146), it is shown that, when the SAW filter having the thickened electrodes formed on the substrate is used, the propagation speed of the surface acoustic waves (or the LSAW) and the propagation speed of the bulk waves (or the SSBW) approach each other if the thickness of the electrodes on the substrate is small, and spurious peaks in the passband of the SAW filter are produced due to the bulk waves. The spurious peaks are detrimental to the steepness of the band-pass characteristics of the SAW filter.
A description will be given of band-pass characteristics of a conventional SAW filter in the above-mentioned literature of M. Ueda et al with reference to FIG. 20.
The conventional SAW filter has a piezoelectric substrate on which aluminum--1% copper electrodes are formed with a given thickness. Specifically, the piezoelectric substrate is the 36.degree.Y-X LiTaO.sub.3 single crystal substrate mentioned above. The crystal of this substrate has X, Y and Z axes and a cut plane, the X axis oriented in a direction of propagation of surface acoustic waves, the Y axis being oblique to a perpendicular line normal to the cut plane of the crystal, the cut plane being rotated around the X axis at the rotated angle of 36.degree. from the Y axis to the Z axis.
The electrodes of aluminum--1% copper alloy which are formed on the substrate are the interdigital transducers (IDT). The thickness of the electrodes is 0.49 .mu.m. This thickness is equivalent to 3% of the wavelength of the excited surface acoustic waves.
As shown in FIG. 20, in the band-pass characteristics of the conventional SAW filter, a spurious peak "A" near the passband of the SAW filter and a spurious peak "B" outside the passband of the SAW filter are produced. The steepness of the band-pass characteristics of the conventional SAW filter is degraded.
In the above conventional SAW filter, the propagation speed of the surface acoustic waves (the LSAW) depends on the thickness of the electrodes on the substrate (or the mass of the electrodes), but the propagation speed of the bulk waves (the SSBW) is independent of the thickness of the electrodes on the substrate. When the frequency band is above the frequency range of some gigahertz, the ratio of the thickness of the electrodes to the wavelength of the excited surface acoustic waves is increased, and the propagation speed of the LSAW relative to the propagation speed of the SSBW is lowered. The passband of the conventional SAW filter in such a case is shifted from the spurious peaks, and the band-pass characteristics of the conventional SAW filter become flattened.
If the ratio of the thickness of the electrodes on the substrate to the wavelength of the excited surface acoustic waves is increased, the propagation loss of the LSAW due to the SSBW is increased. This is the reason why the steepness of the band-pass characteristics of the conventional SAW filter is degraded.
Therefore, in a SAW filter which is operable at desired frequencies of the order of some gigahertz, it is necessary to ensure a certain amount of the thickness of the electrodes on the substrate and decrease the resistance of the IDT. On the other hand, when the conventional SAW filter is used, it is difficult to avoid the increase of the propagation loss of the surface acoustic waves which causes the degradation of the steepness of the band-pass characteristics.
Japanese Patent Application No. 7-265466, which is assigned to the assignee of the present invention, discloses an improved SAW device which is intended to prevent the increase of the propagation loss of the surface acoustic waves. A description will be given of the improved SAW device disclosed in the above patent application.
In the above-mentioned patent application, it has been shown that the operating characteristics of the improved SAW device when it is operated at frequencies of the order of some gigahertz can be made optimum (or the propagation loss can be made minimum) by shifting the rotated angle of the cut plane of the crystal of the substrate to an increased angle greater than the above-mentioned angle 36.degree..
As described above, the effect of the mass of the electrodes when it is operated at frequencies of the order of some gigahertz becomes considerable and not negligible. The wavelength of the excited surface acoustic waves in such a case is extremely small, and the thickness of the electrodes relative to the wavelength of the excited surface acoustic waves cannot be ignored. However, it is shown that, in the improved SAW device, by shifting the rotated angle of the cut plane of the crystal of the substrate to the increased angle greater than the above-mentioned angle 36.degree., the effect of the mass of the electrodes detrimental to the operating characteristics of the SAW device can be eliminated.
In the SAW device of the above-mentioned patent application, the piezoelectric substrate is made of a LiTaO.sub.3 single crystal, the crystal having X, Y and Z axes and a cut plane, the X axis oriented in the direction of propagation of the surface acoustic waves, the cut plane being rotated around the X axis at an increased rotated angle from the Y axis to the Z axis, the increased rotated angle being in a range between 38.degree. and 46.degree.. It is shown that the SAW filter of the above-mentioned patent application achieves a high level of the quality factor Q, and passes the desired high frequencies of the order of some gigahertz with the steepness of the band-pass characteristics.
However, if the rotated angle of the cut plane of the crystal of the substrate is shifted to the increased angle as in the improved SAW device of the above-mentioned patent application, basic parameters which determine operating characteristics of the SAW device, such as the coupling coefficient, the reflection coefficient and other related coefficients, must be changed in accordance with the change of the rotated angle. It is not shown in the above-mentioned patent application what values of the basic parameters are appropriate to obtain optimum operating characteristics of the improved SAW device. The values of the basic parameters which provide optimum operating characteristics of the conventional SAW devices having the 36.degree.Y-X LiTaO.sub.3 substrate are different from those providing the optimum operating characteristics of the improved SAW device. It is desirable to determine appropriate values of the basic parameters of the improved SAW device that attain the optimum band-pass characteristics.